The -300s are ageing aeroplanes. Having said that, they are still busy as passenger shuttles, abused as medium-range tourist transporters (up to 6 hrs even....), and the well maintained ones are still doing scheduled services.
Once they are converted into freighters they usually stay until end of life or until the enthusiastic business plans of unrealistic managers come to a sudden Bank imposed grounding.
Readily available ones are indeed usually not available for long.
You may need to talk to leasing companies, to Boeing or check your companies portfolio, find a half decent -300, get a survey and D-check done, convert it into a freighter.....mind you, 6 months is a tight schedule.